This invention relates to the making of highly aligned polymer films having superior mechanical and thermal properties through a drawing-induced plastic deformation.
More efficient utilization of energy resources requires development of new materials with superior properties, such as mechanical strength or thermal conductivity. For example, bulk polymers usually have low thermal conductivities (˜0.2 W·m−1K−1) as compared to metals (˜40 W·m−1K−1 steel, ˜400 W·m−1K−1 copper). This low thermal conductivity has hindered widespread deployment of polymers in heat transfer applications. As it has beets shown that aligned molecular chains behave like one-dimensional conductors,1 superior thermal and mechanical properties can be achieved through alignment of polymer charm (and filler materials in polymer-based composites).2-9 Due to ability to spin small diameters (5 to 15 μm), which helps to maximize orientation and minimize defects, fibers have emerged as the natural form factor for producing bulk quantities of highly aligned polymeric materials. These fibers can have elastic moduli near the theoretical limit for perfectly aligned crystalline polymer.10 A number of high performance commercially available polyethylene fibers (such as Spectra or Dyneema fiber) have successfully capitalized on processing and synthesis innovations made over the past few decades.11 Fabrication of these high performance fibers typically use a gel spinning technique in which a concentrated polymer gel is first extruded through a small orifice, then simultaneously mechanically drawn and solvent removed—producing highly crystalized, oriented, and strong continuous fibers.5, 10, 12 As opposed to mechanical drawing, Cao et al., used a nano-template to achieve aligned ultra-high molecular weight polyethylene (UHMWPE) nanowires.13 In another approach, Singh et al., fabricated amorphous aligned polythiophene nanofibers during electro-polymerization in nano-templates.14 Other approaches, such as electrospinning, can be used to fabricate large-scale amounts of polymer fibers.15-17 As opposed to gel spinning, however, electrospinning does not lead to highly aligned molecular chains.18 
While fibers are ideal for textiles, however, for practical applications, such as fins in heat exchangers, casings for electronic systems, and biomedical treatments like improved cooling pads for stroke patients, a film form of these materials is essential. The difficulty lies in translating the remarkable material property enhancements seen in high performance fibers into a film form factor. Furthermore, for widespread commercial implementation of these advanced materials, a scalable, continuous, and robust film manufacturing platform is essential. Zone annealing, electrospinning, and melting/drawing are used to fabricate aligned polymer films.19-21 In zone annealing, single crystal mat is locally heated and subjected to tension resulting in an aligned film. Using this method, Kunugi et al., achieved a dynamic modulus of ˜220 GPa at a draw ratio (λ) of ˜175×.20 Zone annealing, however, requires a single crystal mat as the starting material. In comparison, films have been made from polycrystalline polymer by using multi-layer arrays of electrospun nanofibers, but maximum film size appears limited.19 Melting and drawing was used by Langer et al., to produce aligned polymer films.21 In this method, films were fabricated by melting polymer powder between heated quartz plates, then mechanically drawn (λ=1.5×) and annealed. Measured thermal conductivity of these poly (p-phenylene sulfide) films was ˜3 W·m−1K−1 at 200 K. The authors suggested that extrapolating measured thermal conductivity values to room temperature, improvements of approximately two orders of magnitude are possible. As compared to fiber production, this is a low draw ratio as well as a non-continuous process. Also in contrast to electrospinning, zone annealing and melt/drawing are batch scale processes—making them unlikely to be implemented in commercial facilities. At the same time, electrospinning suffers from low molecular chain alignment in the final product.